Is this… IMPLIED NUDITY?! =D
No wait… they’re on the bed in Frame 2. =C
Well, atleast it’s better than my sister. The only underwear that she aims at my head is thongs… hers of course. Like that makes it any better.

Alex: “Well first I use my premotor cortex to plan a the motion of throwing my underwear at you. Then the information is passed to my motor cortex, where the information is then filtered and processed by the basal ganglia and cerebellum. The impulses then activate the lower motor neurons thus moving my arms to propel my boxer shorts at your face.”
Daniel: “I stand corrected. Ew.”

For the undies to have come from the direction the motion blur indicates they must have been thrown by a third party (possibly a surprise attack by an underwear demon). Or Alex’s undies are covered in flubber and he threw them at the wall and they ricocheted onto Dan. I find the second option far more likely.